Vasoactive intestinal peptide is a known, 28-amino acid, carboxy-terminal-amidated peptide hormone. The sequence of human vasoactive intestinal peptide (hereinafter "VIP"), which is the same as that of bovine, porcine and rat vasoactive intestinal peptide, is as follows: EQU H-S-D-A-V-F-T-D-N-Y-T-R-L-R-K-Q-M-A-V-K-K-Y-L-N-S-I-L-N-NH.sub.2,
wherein the "NH.sub.2 " at the carboxy-terminus indicates carboxy-terminal amidation. (See Table A, below, for definitions of the one-letter abbreviations used herein for amino acids).
The presence of VIP has been documented in the neuronal cell structures of the brain, the genito-urinary tract, the tracheobronchial tract and the nasal mucosa, the exocrine glands, including salivary, sweat, pancreas and lacrymal glands, the nerve fibers of many peripheral endocrine organs, the peripheral nervous system, and cells of the gastrointestinal tract.
The numerous and varied biological activities associated with VIP include: (1) neuroregulation; (2) inducement of vasodilation; (3) smooth muscle relaxation; (4) stimulation of the intestinal secretion of water and electrolytes; (5) regulation of water and electrolyte transport in various tissues; (6) inhibition of gastric acid secretion; (7) promotion of glycogenesis; and (8) stimulation of the production of pancreatic juice. As a result of its many biological activities, VIP has a number of potential therapeutic uses.
In connection with its mediation of smooth muscle relaxation, VIP can be utilized in reversing broncheal obstruction due to asthmatic bronchospasm. In vitro and in vivo testing have shown VIP to relax tracheal smooth muscle and protect against bronchoconstrictor agents such as histamine and prostaglandin. Wasserman et al. in Vasoactive Intestinal Peptide, ed, S. I. Said, 177-184, Raven Press, N.Y. 1982; Said et al. Ann N.Y. Acad. Sci. 221, 103-117 (1974); Morice et al., Lancet 1983, ii: 1225-1227; Barnes and Dixon, Am. Rev. Resp. Dis. 130, 162-166 (1984); Morice and Sever, Thorax 39, 707 (1984); Altiere and Diamond, J. Appl. Physiol.: Resp. Env. Ex. Physiol. 56, 986-992 (1984); Altiere and Diamond, Br. J. Pharm. 82, 321-328 (1984); Hand et al., Eur. J. Pharm. 98, 279-284 (1984).
VIP causes bronchodilation when administered intravenously or by inhalation. Intravenous administration has drawbacks, in that the VIP is not limited to specific tissues and has a number of effects other than bronchodilation which are usually considered to be deleterious (i.e. hypotension, tachycardia, flushing). Administration by inhalation is more tissue specific and has fewer side-effects than intravenous administration but appears to be less effective than intravenous administration. Altiere et al., Pharmacologist 25, 123 (1983); Bundgaard et al., Eur. J. Respir. Dis. 64 (Suppl. 128), 427-429 (1983); Barnes and Dixon (1984), supra; Altiere et a)., Chest 86, 153-154 (1984).
The lower efficacy of VIP in bronchodilation when administered by inhalation, in comparison with administration intravenously, is thought to be caused by either (1) rapid degradation of VIP by compounds, including proteolytic enzymes, present in the respiratory tract both in the bronchial airways and the passageways leading thereto (Barrowcliffe et al., Thorax 41, 88-93 (1986)) or (2) limited absorption of VIP through nasal and pulmonary mucosa, due in part to the size of VIP (about 3300 daltons)(Effros and Mason, Am. Rev. Resp. Dis. 127, S59-S65 (1982); Altiere et al. Chest 86 153-154 (1984)) The net effect of these factors is to prevent VIP from reaching its receptor in lung tissue, as it must in order to have a bronchodilating effect. Hence, there exists a need for biologically active analogs of VIP that, in comparison with native VIP, are more resistant to proteolytic and other forms of degradation and are better able to reach VIP receptors, or capable of binding with greater affinity to VIP receptors.
Reduced susceptibility to proteolysis would be an advantageous property of a VIP analog for another reason as well. Such reduced susceptibility would improve the efficiency of producing the analog by recombinant DNA techniques. Microbial or animal host cells employed in syntheses by recombinant DNA techniques often contain proteases that degrade heterologous proteins sought to be synthesized by such techniques and thereby reduce the yields of the desired heterologous protein products.
It would also be desirable to have biologically active VIP analogs that are not amidated at the carboxy-terminus. Such analogs would be easier to make, by both recombinant DNA and chemical techniques, than native VIP or other carboxy-terminal-amidated analogs.
Finally, the naturally occurring VIP has so many biological activities that its use is limited, because beneficial effects are associated unavoidably with significant, deleterious side-effects, especially when the VIP is administered intravenously. Thus, it would be desirable to have analogs with effective doses for particular, desirable biological activities of naturally occurring VIP that are decreased relative to the effective doses for other, undesirable activities of the naturally occurring peptide
Naturally occurring VIP and fowl vasoactive intestinal peptide are disclosed in Said et al., U.S. Pat. Nos. 3,880,826 and 4,016,258 respectively. VIP is a member of the secretin family of peptides, which includes, besides VIP, secretin, glucagon, gastric inhibitory peptide, peptide histidine-isoleucine, peptide histidine-methionine, and the amino-terminal 29 amino acids of growth hormone releasing factor. See e.g., Itoh et al., Nature 304, 547-549 (1983).
A number of VIP analogs are known. See, e.g., Bolin et al., U.S. Pat. No. 4,605,641; Couvineau et al., Biochem. Biophys. Res. Comm. 121, 493-498 (1984); Beyerman et al., Life Sciences 29, 895-902 (1981); Takeyama et al., Chem. Pharm. Bull. 28, 2265-2269 (1980); Gardner et al., Endocrinol. (Japan.) S.R. No. 1, 1-5 (1980); Bodanszky et al., J. Med. Chem. 21, 1171-1173 (1978); and Bodanszky et al., proc. Nat. Acad. Sci. (U.S.A.) 70, 382-384 (1973).
Although some aspects of the interaction of VIP with its receptors have been described (Laburthe et al., Eur. J. Biochem. 139:181 (1984)), virtually no information which describes the relationship between hormone structure and its affinity for binding to receptors exists in the literature.
Analyses of the secondary structures of certain peptide hormones for amphiphilicity have proven useful in the design of analogs of the hormones which bind, with affinities comparable to the natural hormones, to the specific receptors for the hormones. Kaiser et al., Science 223, 249-255 (1984). Such analyses, and design of analogs based on the analyses, have been carried out for growth hormone releasing factor and glucagon. Musso, Ph.D. Dissertation, University of Chicago, Chicago, Ill. U.S.A. (1983); Kaiser et al., supra. It is recognized, however, that analysis of the secondary structure of a peptide hormone for amphiphilicity, while it might suggest design of analogs that bind with high affinity to the hormone's receptors, is not useful in predicting other properties of the hormones, including, most importantly, biological activity and potency, which depend on numerous, ill-understood factors besides receptor affinity.